The present invention relates to an image pickup system which is to be used in combination with a craft, e.g., a spacecraft or an aircraft flying over the ground surface of the earth or similar object zone.
This type of image pickup device is used to remotely sensing an object zone extending along the flight path of a craft. Typical of such an image pickup device are a multi-spectoral scanner (MSS) and a thmatic mapper (TM) to be mounted on LANDSAT which is one of remote sensing spacecrafts developed by NASA (National Aeronautics and Space Administration).
Another and more recent achievement in the remote sensing art is MESSR (Multi-Spectoral Selfscanning Radiometer) disclosed in a paper "TH DEVELOPMENT OF MULTI-SPECTORAL SELFSCANNING RADIOMETER FOR MOS-1", PROCEEDINGS OF THE FOURTEENTH INTERNATIONAL SYMPOSIUM ON SPACE TECHNOLOGY AND SCIENCE, issued in 1984 in Tokyo, pp. 1313-1319. MESSR is an image pickup system of the above-described type and having a linear CCD (Charge Coupled Device) array as a photoelectric detecting device.
All the image pickup systems introduced in the art sense the intensities of visible rays and infrared rays radiated or reflected from an object zone or target zone on the surface of the earth, i.e., radiances. As well known in the art, a radiation flux P incident to this kind of image pickup system and a radiance N representative of the intensity of, for example, light radiated from the target zone have the following relation: EQU N=P/(.omega.S) (1)
where .omega. is the stereoscopic angle of the visual field of the system, and S is the area of the light-sensitive surface of the optics included in the system. In the equation (1), S is a constant particular to the system and, therefore, the incident radiation flux P is proportional to the radiance N. A detector also included in the system is implemented as a photoelectric tube, CCD array or similar photoelectric elements for converting the flux P incident to the optics to an electric signal A. By measuring the amplitude of the electric signal A, it is possible to determine the radiance N of the target zone. Specifically, assuming that the flat area of the land has a standard radiance of 0 dB, it is generally accepted that the radiance changes by about .+-.10 dB from a mountaineous region or similar light region where the radiance is high to a marshy region or similar dark region where the radiance is low. The radiance of the sea is known to be about -10 dB. The cloud is extremely light and often exceeds a radiance of .+-. 20 dB. Moreover, the radiance is susceptible to the season, weather, time, and other various conditions. The image pickup system, therefore, should be provided with an adequate sensitivity for observing such radiance data with accuracy. It is a common practice with this type of image pickup system to transmit a great amount of radiation data from the craft to a terrestrial station by using a digital transmission radio data link which has inherently high communication quality. At this instant, assuming that the digitized data has six bits, the valid dynamic range is 36 dB. To set an adequate sensitivity as mentioned above is to insure the linearity of an amplifier and other analog circuit components, i.e., prevent them from being saturated by input signals to thereby promote the collection of maximum and minimum radiance data without distortions, and to set up levels which can be sufficiently confined in the valid dynamic range of digitized data. For example, assuming the above-mentioned radiance data, the maximum dynamic range is achievable if the sensitivity is 0 dB for the sea and -10 dB for the land.
Since a spacecraft capable of mounting the image pickup system surveys substantially the entire surface of the earth with high resolution, it is caused to fly an orbit at a relatively low altitude which is usually referred to as a polar orbit or solar synchronous semirecurrent orbit. Such an orbit may have an altitude of 900 km, an orbit inclination angle of 99.degree., and an orbital period of 103 minutes by way of example. The period of time of transmission available with the radio data link is limited to the period of time for which the spacecraft is visible. In the specific case described above, the visible period of time, i.e., the transmittable period of time is about 15 minutes per orbit.
It has been customary with image pickup systems, including MSS, TM and MESSR, to manipulate the sensitivity of the system by external control in matching relation to a change in lightness or radiance, e.g., on the transition of target area from the land to the sea. Specifically, all the conventional systems have their sensitivity controlled by a command sent from the land. A terrestrial station, or command and data acquisition station, sets an adequate sensitivity estimated on the basis of positional information on the scheduled orbit of the spacecraft and information indicative of the kind of a region being surveyed, e.g., whether the region is a flat region or a mountaineous region of the land or whether it is the sea. Data representative of the adequate sensitivity is sent to the spacecraft as a common while the spacecraft uses it as a control signal for controlling the image pkckup system.
The conventional image pickup systems cannot adjust sensitivity by themselves and are fixed at a sensitivity set up by externally derived control data, as stated above. This brings about a problem that on the noticeable transition of the subject of survey from the sea to the land or the transition from fine weather to rainy weather, the dynamic range associated with change in radiance is broadened, resulting in the increase in bit length, i.e., in the amount of data to be transmitted in the event of quantization.
On the other hand, when the bit length, i.e., the amount of data to be transmitted is fixed, the effective dynamic range associated with the change in the radiance of a region is reduced. Then, the degree of identification of a region is lowered due to non-linearity, i.e., since saturation and omission occur in a light portion and a dark portion, respectively.
Moreover, to control the sensitivity from the land, it is necessary to estimate the radiance of the area of interest and the duration thereof beforehand. This, coupled with the fact that the control over sensitivity should be completed in the limited transmittable period of time, makes it difficult to execute real-time confirmation and evaluation of the result of control, again resulting in inaccurate and inadequate control.